Method and system to locate a storage device

ABSTRACT

A system, to locate at least two storage devices from among a plurality of storage devices, receives a request for the data item. The request includes a data identifier for the data item. Next, the system generates a start number and a step number based on the data identifier. The system locates a first storage device utilizing the start number and, if the first storage device is available, the system reads the data item from the first storage device. However, if the first storage device is unavailable, the system utilizes the step number and the start number to compute a backup number that is utilized to locate a second storage device. If the second storage device is available, the system reads the data item from the second storage device.

FIELD OF THE INVENTION

An embodiment relates generally to the technical field of data storageand, in one exemplary embodiment, to a method and system to locate astorage device.

BACKGROUND OF THE INVENTION

Systems that store large quantities of data typically utilize multiplestorage devices because multiple storage devices address the issues ofavailability and scalability. For example, data may not be available ifstored on a single storage device that fails (e.g., cannot be accesseddue to a hardware or software failure). Multiple storage devices may beutilized to overcome the lack of availability by providing a backupstorage device that may be accessed if a primary storage device fails.In addition, an organization tends to require more storage capacity overtime and the incremental addition of storage devices (scaling) may bepreferable to expanding the capacity of a single storage device. Thus,multiple storage devices may be utilized to overcome the issues ofscalability and availability. Nevertheless the utilization of multiplestorage devices poses issues of load balancing and affinity.

SUMMARY OF THE INVENTION

According to one aspect there is provided a method to locate at leasttwo storage devices from among a plurality of storage devices. Themethod includes receiving a request for a data item, the requestincluding a data identifier for the data item; generating a start numberand a step number based on the data identifier; locating a first storagedevice utilizing the start number; if the first storage device isavailable, reading the data item from the first storage device; however,if the first storage device is unavailable, utilizing the step numberand the start number to compute a backup number that is utilized tolocate a second storage device; and if the secondary storage device isavailable, reading the data item from the second storage device.

According to another aspect there is provided a method to locate atleast two storage devices to store a data item, the method includesreceiving a request to store the data item, the request including a dataidentifier for the data item; generating a start number and a stepnumber based on the data number; locating a first storage device, of afirst set of storage devices, based on the start number; if the firststorage device is available, storing the data item on the first storagedevice; and adding the step number to the start number to compute abackup number that is utilized to locate a second storage device, of asecond set of storage devices, to store a backup copy of the data item,the backup number selected to evenly distribute respective backup copiesof the data item over the second set of storage devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements and in which:

FIG. 1 is a network diagram depicting a system, according to oneexemplary embodiment of the present invention;

FIG. 2 is a network diagram depicting software and hardware componentsof the system, according to one exemplary embodiment of the presentinvention;

FIG. 3A is a block diagram illustrating databases, according to anexemplary embodiment of the present invention;

FIG. 3B is a block diagram illustrating a double-hash algorithm,according to an exemplary embodiment of the present invention;

FIG. 4 is an interactive flow chart illustrating a method, according toan exemplary embodiment of the present invention, to utilize a userprofile to personalize a web page;

FIG. 5 is an interactive flow chart illustrating a method, according toan exemplary embodiment of the present invention, to locate storagedevices to read a data item;

FIG. 6 is an interactive flow chart illustrating a method, according toan exemplary embodiment of the present invention, to locate storagedevices to store a data item;

FIGS. 7-8 illustrate user interface screens, according to an exemplaryembodiment of the present invention;

FIG. 9 is a block diagram illustrating a trading system, according to anexemplary embodiment of the present invention;

FIG. 10 is a block diagram illustrating multiple marketplace and paymentapplications that, in one exemplary embodiment of the present invention,are provided as part of the network-based trading platform;

FIG. 11 is a high-level entity-relationship diagram, illustratingvarious tables that are utilized by and support the network-basedtrading platform and payment applications, according to an exemplaryembodiment of the present invention; and

FIG. 12 illustrates a diagrammatic representation of a machine, in theexemplary form of a computer system, within which a set of instructions,for causing the machine to perform any one or more of the methodologiesdiscussed herein, may be executed.

DETAILED DESCRIPTION

A method and system to locate a storage device are described. In thefollowing description, for purposes of explanation, numerous specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be evident, however, to one skilled inthe art that the present invention may be practiced without thesespecific details.

Load Balancing and Affinity

Load balancing may be concerned with locating a storage device toinitially store a data item so that many data items will be distributedacross a set of storage devices and affinity is concerned withsubsequently retrieving the data item from the storage device. Forexample, an initial request to store a data item requires that a systemlocate a storage device from multiple storage devices to store the dataitem. In addition, a subsequent request for the data item requires thesystem to locate the correct storage device to retrieve the data item.These issues have been addressed with a number of schemes. One schemeutilizes a modulo to locate a storage device to store a data item. Forexample, assume a data item is associated with a unique identifier thatranges from 1 to 1000 and also assume that a system includes 10 storagedevices. Such a system may utilize a modulo 10 to locate the storagedevice to store the data item. Simply put, the unique identifier isdivided by the modulo and the remainder is utilized to locate the properstorage device. A disadvantage of this scheme is that adding or removingstorage devices may require halting the system to recompile softwaremodules or to update the appropriate applications with the new modulo.Another scheme may utilize a round robin approach that randomly locatesstorage devices. This scheme may store information (e.g., identifyingthe storage device, identifying the data item, associating the storagedevice with the data item) in a database to enable the retrieval of thedata item. A disadvantage of this scheme is that removing storagedevices may also require halting the system to update the databasefiles. Other schemes may utilize other approaches (e.g., locate thestorage device with the least number of connections, locate the storagedevice with the least number of data items, etc.); however, they alsoexhibit the above-described disadvantages.

The issues of load balancing and affinity typically also need to beaddressed if a storage device fails. For example, as described above, adata item is typically stored on two storage devices, the secondarystorage device providing a backup copy of a data item. One approach maybe to dedicate a secondary storage device to exclusively backup anotherstorage device. This approach is expensive because it doubles the numberof storage devices in a system. Another approach may be to utilize astorage device that dedicates a portion of capacity to backup support.This approach requires the reservation of sufficient capacity to preventoverloading. Both approaches fail to balance the load of a failedstorage device over the remaining storage devices.

In general, embodiments described below feature a system that providespersonalization services to an application that personalizes a web pagebefore communicating the web page to a user. The system providespersonalization services by maintaining a user profile for each user inthe system that is identified with a globally unique identifier (GUID).The user profile uniquely characterizes the user based, for example, onprevious selections by the user, demographic information about the user,and other stored information that is personal to the user. The userprofile is stored in at least two storage devices (e.g., caches), afirst or primary storage device and a second or secondary storagedevice. Dual storage is utilized to sustain personalization servicesnotwithstanding a single point of failure that prevents access to one ofthe storage devices. Further, the two copies may be stored on any twostorage devices in a system that includes N storage devices. Forexample, a sequence of storage devices may be determined based theoutput of a double-hash algorithm that receives as input an GUID thatcorresponds to a user profile and a parameter that specifies the maximumnumber of storage devices (MNSD) in the system. The sequence of thestorage devices has the property that each storage device in the systemappears in the sequence without repetition. The first two storagedevices that are determined to be available are utilized to store theuser profile. Thus, a storage device may be removed from the system oradded to the system (e.g., up to N storage devices) without halting thesystem to reconfigure configuration files or to recompile applicationsbecause a search for backup storage devices continues until the maximumnumber the storage devices (MNSD) are exhausted. The user profile issimply stored on and retrieved from the first two available storagedevices. In addition, the above scheme backs up a primary storage deviceby evenly distributing a backup copy of the user profiles to theremaining storage devices. For example in a system of 5 storage devices,the above scheme may evenly distribute the user profiles of a primarystorage device to each of the four secondary storage devices in thesystem (e.g., each of the secondary storage device stores 25% of thedata items that are stored as primary storage on the primary storagedevice). Thus, a primary storage device failure results in evenlydistributing the user profiles over all of the secondary storagedevices.

More specifically, an exemplary system may function in the followingmanner. A user makes a selection on a web page that generates a requestfor the user's user profile. The request further includes an GUID thatis associated with and identifies the user. The GUID and the maximumnumber of storage devices (MNSD) are provided as parameters to a doublehash algorithm that generates a start number and a step number. Thestart number is utilized to locate a primary storage device. If theprimary storage device is available, then the user profile is read fromthe primary storage device. Otherwise the start number is added to thestep number to generate a location number that can be used to identify asecondary storage device. If the secondary storage device is availablethen the user profile is read from the secondary storage device. If thesecondary storage device is not available, the step number is added tothe location number to generate a new location number that can beutilized to identify a further secondary storage device. The step numbercontinues to be added to the previously computed location number untilthe storage devices are exhausted or an available secondary storagedevice is located. Accordingly, the double-hash algorithm generatesstart and step numbers that are used to generate a sequence of locationnumbers for respective storage devices. The sequence of location numbersidentify a sequence of storage devices without specifying the samestorage device twice (e.g., the step number is relatively prime to theMNSD). One embodiment may utilize an MNSD that is larger than the actualinstalled number of storage devices, thereby facilitating the additionof storage devices to the system (e.g., scaling) without halting thesystem for recompilation of software or reconfiguration of configurationfiles. For example, a system may include an MNSD with a value of fivealthough the system may also include only three storage devices that areinstalled. During operation, the generated search sequences will includestorage device numbers that identify the two uninstalled storage devices(e.g., storage device 4 and 5), each with an unavailable status. Thus,the two storage devices may not be available for the storage orretrieval of user profiles; nevertheless, if additional capacity isrequired then up to two storage devices may be added to the systemwithout impact.

FIG. 1 is a network diagram depicting a system 10, according to oneexemplary embodiment of the present invention, having a client-serverarchitecture. Platforms, in the exemplary form of an information storageand retrieval platform 12, provides server-side functionality, via anetwork 14 (e.g., the Internet) to one or more clients 17, 19, 21. FIG.1 illustrates multiple information storage and retrieval platforms 12communicating over a wide geographical region, for example, aninformation storage and retrieval platform 12 is located at Dallas, SanFrancisco, and New York, 11, 13 and 15. Each information storage andretrieval platform 12 includes a profile client 40 and a profile manager44. The profile client 40 at a specific information storage andretrieval platform 12 may request personalization services from aprofile manager 44 at any storage and retrieval platform 12. Forexample, the profile client 40 at Dallas 11 may request the profilemanager 44 at Dallas, San Francisco or New York 11, 13, or 15 to returna user profile 54 from a storage device 46 that is associated with theprofile manager 44 at the specific location. Further, while the system10 shown in FIG. 1 employs a client-server architecture, the presentinvention is of course not limited to such an architecture, and couldequally well find application in a distributed, or peer-to-peer,architecture system.

FIG. 2 is a network diagram depicting software and hardware components20, according to one exemplary embodiment, for the system 10. FIG. 2illustrates, for example, a web client 16 (e.g., a browser, such as theINTERNET EXPLORER browser developed by Microsoft Corporation of Redmond,Wash. State), and a programmatic client 18 executing on respectiveclient machines 20 and 22. In addition, the client machines 20 and 22include a cookie 23 that includes a data identifier or data number, inthe exemplary form of a globally unique identifier 25 (GUID). Apersonalization service may drop the cookie 23 that contains the GUID 25to the client machine 20 or 22 to uniquely identify a user that operatesthe respective client machine 20 or 22.

Turning specifically to the information storage and retrieval platform12, an application program interface (API) server 24 and a web server 26are coupled to, and provide programmatic and web interfaces respectivelyto, one or more application servers 28 that host applications 38. Theweb client 16, it will be appreciated, accesses the applications 38 viathe web interface 26 supported by the web server 26. Similarly, theprogrammatic client 18 accesses the various services and functionsprovided by the applications 38 via the programmatic interface providedby the API server 24. The applications 38 may invoke a profile client 40to obtain personalization services. As described above, the profileclient 40 may provide personalization services by communicating with alocal profile manager 44 (e.g., San Francisco 13) or by communicatingwith a remote profile manager 44 (e.g., Dallas, or New York, 11 and 15).Specifically, the personalization services performed by the profilemanager 44 may include reading a data item, in the exemplary form of auser profile 54, from and/or storing a user profile 54 to a storagedevice 46 (e.g., cache) at the respective site (e.g., San Francisco,Dallas, or New York, 13, 11 and 15). To this end, the profile client 40includes a receiving module 50 and a processing module 52 that mayaccess a cluster map 42 to determine whether the profile manager 44 at arespective site 11, 13, and 15 is available (e.g., the profile client 40may actively communicate with the profile manager 44 over an HTTPconnection).

The processing module 52 allocates GUIDs and identifies primary andsecondary storage devices 46. The processing module 52 allocates an GUIDin response to a first request from a user for a user profile. Theprocessing module 52 communicates the GUID 25 to the client machine(e.g., client machines 22, 20) that stores the GUID 25 in a cookie 23 ona local storage device of the client. Thereafter, the client machine 22,20 requests the user profile by including the allocated GUID. The GUID25 generated by the processing module 52 is guaranteed to be unique(e.g., guaranteed unicity across space and time). The processing module52 generates the GUID 25 by combining the address of a networking cardthat is associated with a specific application server 28, a start time,and a counter of previously allocated GUIDs 25. In addition, theprocessing module 52 utilizes a double hash algorithm to identify orlocate primary and secondary storage devices 46 as described in FIG. 3B.

The back servers 30 are shown to include the previously describedprofile manager 44 and the storage device 46 (e.g., cache) which mayinclude one or more user profiles 54. Each user profile 54 includesinformation pertaining to a particular user, as described further below.The back end servers 30 facilitate access to one or more databaseservers 34 that are utilized to access databases 36. The databases 36are illustrated to include one or more persistent user profiles 54 whichmay be non-volatile copies of the one or more user profiles 54 thatreside in the storage device 46 on the back end servers 30.

FIG. 3A is a block diagram illustrating a user profile 54, a storagedevice 46, and a cluster map 42, according to an exemplary embodiment.The user profile 54 includes a globally unique identifier (GUID) 25,user attributes 56, personalization attributes 58, session attributes 60and persistent attributes 65. The GUID 25 uniquely identifies a singleuser of the system 10 and is utilized to identify a user profile 54.

The user attributes 56, personalization attributes 58, sessionattributes 60 and persistent attributes 65 may each contain attributes67 (not illustrated). Each attribute may be a datum of information abouta user (e.g., past selection, purchasing history, demographicinformation, geographic information, age, gender, prior web pageselection, elapsed time for current session, etc.). The user attributes56 are attributes that may be copied from existing databases 36 for aparticular user. For example, the databases 36 may include a purchasinghistory database that includes recent purchases of the user that theprofile manager 44 copies into the user profile 54. Other embodimentsmay include a preferences database that characterizes user preferences.The personalization attributes 58 are information inserted or modifiedby off line tools that utilize personalization rules, batch jobs, etc.The personalization attributes 58 and the user attributes 56 are readonly attributes and may not be modified by the applications 38.

The session attributes 60 are transient attributes kept in the profilefor the duration of a session only. A session may begin when theinformation storage and retrieval platform 12 receives a request from auser that is operating a client machine 22, 20. The session may endafter a period of time elapses without detecting user activity (e.g., amouse click). For example, in one embodiment the period of time may befive minutes. The session attributes 60 may be contrasted with thepersistent attributes 65 which are written to the database 36 andavailable from one session to another.

The storage device 46 (e.g., cache) is utilized to store one or moreuser profiles 54. The storage device 46 includes a storage device number77, a maximum number of storage devices 79 (MNSD), and a user profilestorage area 61. The storage device number 77 uniquely identifies aparticular storage device 46 in the system 10. For example, FIG. 1illustrates three information storage and retrieval platforms 12 (SanFrancisco, New York, and Dallas) each of which may include a storagedevice 46 that is uniquely identified or located with a storage devicenumber 77 (e.g., 1, 2, and 3). FIG. 1 read in view of FIG. 2 furtherillustrates a single storage device 46 at information storage andretrieval platforms 11, 13 and 14; however, it will be appreciated thatother embodiments may include multiple storage devices 46 for eachinformation storage and retrieval platform 12. The MNSD 79 may definethe maximum number of storage devices for the system 10 and is utilizedby the double hash algorithm. Typically the MNSD 79 number may begreater than the actual number of storage devices 46. For example, FIG.1 read in view of FIG. 2 illustrates a system 10 with storage devices 46at San Francisco, New York and Dallas (e.g., identified with storagedevice numbers 77, namely 1, 2 and 3). Nevertheless, the MNSD 79 forsuch a system 10 may be greater than 3, for example 5, as describedabove.

The user profile storage area 61 is utilized to store and retrieve(e.g., read) user profiles 54. A user profile 54 may be stored in theuser profile storage area 61 if the storage device 46 is identifiedsolely with a start number or if the storage device 46 is identifiedwith the aid of a step number. If the user profile 54 is identifiedsolely with the start number, then primary storage capacity is utilizedand if the user profile 54 is identified with a start number and a stepnumber then secondary storage capacity is utilized.

The cluster map 42 includes multiple storage device entries 72. Eachstorage device entry 72 includes a status 74 and connection information76. The storage device entry 72 corresponds to a particular storagedevice 46 that may reside on a back end server 30 at a particularinformation storage and retrieval platform 12. The profile client 40reads the status 74 to determine if the storage device 46 is availablefor storing or reading user profiles 54. For example, in one embodiment,the connection information 76 may include HTTP connection informationthat is utilized by the profile client 40 to connect to and communicatewith the profile manager 44 at the respective information storage andretrieval platforms 12. The profile managers 44 maintain the cluster map42 and communicate an up-to-date cluster map 42 to the profile clients40 on the application servers 28.

FIG. 3B is a block diagram illustrating a double-hash algorithm 43,according to an exemplary embodiment, that is executed by a processingmodule 52. The double-hash algorithm 43, as previously stated, isutilized to identify primary and secondary storage devices 46. Thedouble-hash algorithm 43 receives inputs in the form of an GUID 25 andan MNSD 79 and generates outputs in the form of a start number and astep number. The start and step numbers are intermediate values that areutilized to identify primary and secondary storage devices 46. Forexample:

-   Double Hash (GUID, MNSD)=step number, start number-   Input: GUID=1, MNSD=5;-   Output: start number=1, step number=1;

Recall that the start number and step number are further utilized toproduce a search sequence to locate an available storage device 46. Forexample, the profile client 40 may determine whether a storage device 46is available by first computing a search sequence composed of successivestorage device numbers 77 as follows: start number, start number+(1×stepnumber), start number+(2×step number); start number+(3×step number);number+(4×step number). Consequently, the profile client 40 may searchfor an available storage device 46 with the following search sequence:“1, 2, 3, 4 and 5” (e.g., start number=1, step number=1, MNSD=5). Notethat the storage device 46 that is identified with the storage devicenumber 77 of “1” is computed solely with the start number and maytherefore be deemed a primary storage device because primary storagecapacity may be utilized. All of the other storage device numbers 77 arecomputed with the aid of the step number and accordingly are deemedsecondary storage devices because secondary storage capacity may beutilized. Accordingly, if the primary storage device 46 is availablethen the user profile 54 may be read from or written to the user profilestorage area 61 of the primary storage device 46. However, if theprimary storage device 46 is not available then the user profile 54 maybe read from or written to the user profile storage area 61 of asecondary storage device 46 (e.g., 2, 3, 4 or 5).

One embodiment of the double-hash algorithm 43 may be utilized togenerate start and step numbers that utilize 50% of the user profilestorage area 61 as a primary storage device and 50% of the user profilestorage area 61 as a secondary storage device. Other embodiments of thedouble-hash algorithm may utilize other ratios.

The double-hash algorithm 43 may exhaust the primary storage areacapacity of one storage device 46 before utilizing the primary storagearea capacity of another storage device 46. For example, consider thefollowing GUID 25 allocations: A, B, C, D, and E. The double hashalgorithm may compute the following start and step numbers:

-   Double Hash (GUID, MNSD=5)=start number, step number

GUID Start number Step number A 1 1 B 1 2 C 1 3 D 1 4 E 1 1

Note that the primary storage capacity associated with the storagedevice 46 that is located with the start number one is utilized orallocated before the primary storage capacity of another storage device46. Also note that the above allocation scheme ensures that the userprofiles 54 that are stored on the storage device 46 that is locatedwith the start number one will be evenly distributed in the remainingstorage devices 46.

FIG. 4 is an interactive flow chart illustrating a method 78, accordingto an exemplary embodiment, to utilize a user profile to personalize aweb page. The method 78 illustrates a client machine 20 on the left andservers on the right. The servers include an application server 28 atSan Francisco, a back end server 30 at San Francisco, and a back endserver 30 at New York.

At the client machine 20, at operation 80, a user operating a clientmachine 20 selects a user interface element on a web page. FIG. 7illustrates a user interface 81, according to an exemplary embodiment.The user interface 81 illustrates a web page to browse categories on theXYZ Information Storage and Retrieval System. The user selects a bookscategory 84 at the client machine 20.

Returning to FIG. 4, at operation 82, an application 38 at theapplication server 28 in San Francisco 13 receives the request andinvokes a profile client 40 to locate and return the user profile 54associated with the user at the client machine 20. It will beappreciated that the application 38 may be one of many applications 38that may utilize personalization services by invoking the profile client40. At decision operation 84, the profile client 40 determines if anGUID 25 was communicated with the request for the user profile 54. If anGUID 25 was communicated, then a branch is made to operation 88.Otherwise, a branch is made to operation 90.

At operation 90, the profile client on the application server 28 at SanFrancisco 13 receives a request to allocate an GUID 25. The profileclient 40 allocates the GUID 25 and communicates the GUID 25 to the webclient 16 on the client machine 20, the web client 16 persistentlystoring the GUID in a cookie (operation 92). Henceforth, the clientmachine 20 communicates the GUID 25 to the information storage andretrieval platform 12 with each subsequent user interface selection.

At operation 88, the profile client 40 locates a storage device 46 andreads the user profile 54 from the storage device 46. FIG. 5 is aninteractive flow chart illustrating a method 94, according to anexemplary embodiment, to locate at least two storage devices from aplurality of storage devices to read a data item (e.g., the user profile54). The method 94 illustrates at left, middle, and right an applicationserver 28 at San Francisco, a back end server 30 at San Francisco, and aback end server 30 at New York, respectively.

At operation 96, the receiving module 50 receives a request for the userprofile 54, the request including the GUID 25 associated with the userat the client machine 20. At operation 98, the processing module 52generates a start number and a step number by executing the double hashalgorithm with the GUID 25 associated with the user and the MNSD 79. Atoperation 100, the processing module 52 utilizes the start number tolocate the primary storage device 46 by utilizing the step number toindex to the appropriate storage device entry 72 in the cluster map 42.At decision operation 102, the processing module 52 determines if theprimary storage device 46 is available by utilizing the status 74. Ifthe processing module 52 determines that the primary storage device 46has an available status 74 then a branch is made to operation 104.Otherwise, a branch is made to operation 106.

At operation 104, the processing module 52 communicates a request,including the GUID 25, to the profile manager 44 at San Francisco 13 toread the user profile 54 from the storage device 46 (e.g., cache).

At operation 105, the profile manager 44 at San Francisco 13 receivesthe request and reads the user profile 54 from the user profile storagearea 61 of the storage device 46 (e.g., cache). If the profile manager44 determines that the user profile 54 is not in the storage device 46(e.g., cache) then the profile manager 44 requests the correspondingpersistent user profile 54 from the database servers 34 that, in turn,reads the corresponding persistent user profile 54 from the database 36.Next, the profile manager 44 at San Francisco 13 communicates the userprofile 54 back to the profile client 40 at San Francisco 13 andprocessing continues at operation 107. At operation 107, the profileclient 40 returns the user profile 54 to the application 38 andprocessing continues on FIG. 4 at operation 120.

At operation 106, the processing module 52 on the application server 28at San Francisco 13 adds the step number to the start number to computea back up number that is utilized to locate a secondary storage device.The processing module 52 utilizes the backup number to index to theappropriate storage device entry 72 in the cluster map 42 (operation108).

At decision operation 110, the processing module 52 determines if thesecondary storage device 46 is available by utilizing the status 74. Ifthe status 74 indicates that the secondary storage device 46 isavailable then a branch is made to operation 111. Otherwise, a branch ismade to operation 112.

At operation 111, the processing module 52 communicates a request to theprofile manager 44 at New York 15 to read the user profile 54 from thesecondary storage device 46.

At operation 113, the profile manager 44 at New York 15 receives therequest and reads the user profile 54 from the storage device 46 (e.g.,cache). If the profile manager 44 determines that the user profile 54 isnot in the storage device 46 (e.g., cache) then the profile manager 44requests the corresponding persistent user profile 54 from the databaseservers 34 that, in turn, reads the corresponding persistent userprofile 54 from the database 36. Next, the profile manager 44 at NewYork 15 communicates the user profile 54 back to the profile client 40at San Francisco 13.

At operation 112, the processing module 52 on the application server 28at San Francisco 13 adds the step number to the backup number to computea new back up number. At decision operation 114, the processing module52 determines if the back up number is greater than the maximum numberof storage devices 79 (MNSD). If the back up number is greater than theMNSD 79 then a branch is made to operation 116. Otherwise, a branch ismade to decision operation 118.

At operation 116, the processing module 52 modulos the back up numberwith the maximum number of storage devices (MNSD) and saves the resultin the backup number.

At decision operation 118, the processing module 52 determines if theback up number is equivalent to the start number. If the back up numberis equivalent to the start number, the method 94 ends and processingcontinues at operation 120 on FIG. 4. Otherwise, the method 94 branchesto box number operation 108.

Returning to FIG. 4, at operation 120, the application 38 modifies theuser profile 54 based on the user's selection of the books category fromthe browse web page. At operation 122, the application 38 invokes theprocessing module 52 to locate a storage device 46 (e.g., cache) tostore the user profile 54. FIG. 6 illustrates a method 124, according toan exemplary embodiment, to locate at least two storage devices to storea data item. At operation 126, the receiving module 50 receives arequest to store a plurality of copies of a data item (e.g., userprofile 54).

At operation 128, the processing module 52 utilizes the double hashalgorithm to generate a start number and a step number based on the GUID25 and the MNSD 79. At operation 130, the processing module 52 locatesthe primary storage device 46 by utilizing the start number to indexinto the appropriate storage device entry 72 in the cluster map 42. Atoperation 132, the processing module 52 determines if the primarystorage device is available by utilizing the status 74 in theappropriate storage device entry 72 in the cluster map 42. If the status74 indicates the primary storage device is available then a branch ismade to operation 136. Otherwise, a branch is made to operation 134.

At operation 134, the processing module 52 initializes a storage counterto zero. The storage counter is utilized to count the number of storagedevices that are written into by the processing module 52.

At operation 136, the processing module 52 requests the profile manager44 on the back end server 30 at San Francisco to store the user profile54. The request includes a copy of the user profile 54 (e.g., updated bythe application 38 ), the GUID 25. Next the processing module 52initializes the storage counter to one thereby registering a first writeof the user profile 54.

At operation 147, the profile manager 44 on the backend server 30 at SanFrancisco 13 stores the user profile in the storage device 46 (e.g.,cache).

At operation 140, the processing module 52, at the application server 28at San Francisco 13, adds the step number to the start number to computea back up number. Next, the processing module 52 locates the secondarystorage device 46 based on the back up number (operation 142) and, atdecision operation 144, determines if the located secondary storagedevice is available. The processing module 52 utilizes the appropriatestorage device entry 72 in the cluster map 42 and reads the status 74 tomake this determination. If the secondary storage device 56 isavailable, a branch is made to operation 146. Otherwise, a branch ismade to operation 148.

At operation 146, the processing module 52 communicates a request tostore the data item on the storage device. The request includes a copyof the user profile 54 that has been updated by the application 38 andthe GUID 25. Next, the processing module 52 increments the storagecounter by one thereby registering a write of the user profile 54.

At decision operation 152, the processing module 52 determines if thestorage counter equals two. If the storage counter equals two then themethod 124 ends. Otherwise, a branch is made to operation 148.

At operation 148, the processing module 52 adds the step number to theback up number to compute a new back up number. Next, the processingmodule 52 determines if the back up number is greater than the maximumnumber of devices (operation 154). If the back up number is greater thanthe maximum number of devices, a branch is made to operation 156.Otherwise, a branch is made to decision operation 158. At operation 156,the processing module 52 modulos the back up number with the MNSD 79.

At decision operation 158, the processing module 52 determines if theback up number is equal to the start number. If the backup number is notequal to the start number then a branch is made to operation 142. If thebackup number is equal to the start number then the storage devices 46have been exhausted, the method 124 ends, and processing continues atoperation 160 on FIG. 4.

Returning to FIG. 4 at operation 160, the application 38 personalizesthe web page based on the user profile 54 and communicates the web pageto the client machine 20. At operation 162, the user receives thepersonalized web page as illustrated on FIG. 8.

FIG. 8 illustrates a user interface 164, according to an exemplaryembodiment, to browse books. The user interface 164 includesbook-browsing categories 168 and is personalized in the form ofsuggestions 170 for John Doe, the user operating the client machine 20.The application 38 generated the suggestions 170 by utilizing the userprofile 54.

Returning to FIG. 4, at operation 172, a period of time has elapsed(e.g., 5 minutes) after the users last selection at the client machine20 thereby triggering the profile manager 44 to communicate the userprofile 54 to the database servers 34 that, in turn, stores the userprofile to the database 36. At operation 174, a similar timeout occurswith respect to the profile manager 44 at the back end server 30 at theNew York 15 and a similar operation is performed.

Network Based Trading Platform Embodiment

The above-described invention may be embodied in any system thatrequires the storage and retrieval of data items. For example, theinvention may be embodied in a network-based trading platform 230 asdescribed below.

FIG. 9 is a network diagram depicting a system 232, according to oneexemplary embodiment of the present invention, having a client-serverarchitecture. A commerce platform, in the exemplary form of anetwork-based trading platform 230, provides server-side functionality,via a network 234 (e.g., the Internet) to one or more clients. FIG. 9illustrates, for example, a web client 236 (e.g., a browser, such as theINTERNET EXPLORER browser developed by Microsoft Corporation of Redmond,Wash. State), and a programmatic client 238 executing on respectiveclient machines 240 and 242.

Turning specifically to the network-based trading platform 230, anApplication Program Interface (API) server 244 and a web server 246 arecoupled to, and provide programmatic and web interfaces respectively to,one or more application servers 248. The application servers 248 hostone or more marketplace applications 250 and payment applications 252and a profile client 40. The application servers 248 are, in turn, shownto be coupled to one or more back end servers 30 that include a profilemanager 44 and a storage device 46 that operate to providepersonalization services as previously described. The storage device 46also includes a user profile 54 that may be persistently stored to thedatabase 256, as previously described. The back end servers 30 are, inturn, shown to be coupled to one or more databases servers 254 thatfacilitate access to one or more databases 256.

The marketplace applications 250 provide a number of marketplacefunctions and services to users that access the network-based tradingplatform 230. The payment applications 252 likewise provide a number ofpayment services and functions to users. The payment applications 252may allow users to quantify for, and accumulate, value (e.g., in acommercial currency, such as the U.S. dollar, or a proprietary currency,such as “points”) in accounts, and then later to redeem the accumulatedvalue for products (e.g., goods or services) that are made available viathe marketplace applications 250. While the marketplace applications 250and payment applications 252 are shown in FIG. 9 to both form part ofthe network-based trading platform 230, it will be appreciated that, inother embodiments, the payment applications 252 may form part of apayment service that is separate and distinct from the network-basedtrading platform 230.

Further, while the system 232 shown in FIG. 9 employs a client-serverarchitecture, the present invention is of course not limited to such anarchitecture, and could equally well find application in a distributed,or peer-to-peer, architecture system. The various marketplace andpayment applications 250 and 252 could also be implemented as standalonesoftware programs, which do not necessarily have networkingcapabilities.

The web client 236, it will be appreciated, accesses the variousmarketplace and payment applications 250 and 252 via the web interfacesupported by the web server 246. Similarly, the programmatic client 238accesses the various services and functions provided by the marketplaceand payment applications 250 and 252 via the programmatic interfaceprovided by the API server 244. The programmatic client 238 may, forexample, be a seller application (e.g., the TURBOLISTER applicationdeveloped by eBay Inc., of San Jose, Calif.) to enable sellers to authorand manage listings on the network-based trading platform 230 in anoff-line manner, and to perform batch-mode communications between theprogrammatic client 238 and the network-based trading platform 230.

FIG. 9 also illustrates a third party application 258, executing on athird party server machine 260, as having programmatic access to thenetwork-based trading platform 230 via the programmatic interfaceprovided by the API server 244. For example, the third party application258 may, utilizing information retrieved from the network-based tradingplatform 230, support one or more features or functions on a websitehosted by the third party. The third party website may, for example,provide one or more promotional, marketplace or payment functions thatare supported by the relevant applications of the network-based tradingplatform 230.

Marketplace and Payment Applications

FIG. 10 is a block diagram illustrating multiple marketplaceapplications 250 and payment applications 252 that, in one exemplaryembodiment, are provided as part of the network-based trading platform230. The marketplace applications 250 and payment applications mayutilize a profile client 40 (not illustrated) to obtain personalizationservices. For example, the personalization services may include storageand retrieval of a user profile 54 (not illustrated) that is utilized bythe marketplace applications 250 or payment applications 252 topersonalize a web page as previously described with respect to system10.

Turning to the illustrated marketplace applications 250 and paymentapplications 252, the network-based trading platform 230 may provide anumber of listing and price-setting mechanisms whereby a seller may listgoods or services for sale, a buyer can express interest in or indicatea desire to purchase such goods or services, and a price can be set fora transaction pertaining to the goods or services. To this end, themarketplace applications 250 are shown to include one or more auctionapplications 262 which support auction-format listing and price settingmechanisms (e.g., English, Dutch, Vickrey, Chinese, Double, Reverseauctions etc.). The various auction applications 262 may also provide anumber of features in support of such auction-format listings, such as areserve price feature whereby a seller may specify a reserve price inconnection with a listing and a proxy-bidding feature whereby a biddermay invoke automated proxy bidding.

A number of fixed-price applications 264 support fixed-price listingformats (e.g., the traditional classified advertisement-type listing ora catalogue listing) and buyout-type listings. Specifically, buyout-typelistings (e.g., including the Buy-It-Now (BIN) technology developed byeBay Inc., of San Jose, Calif.) may be offered in conjunction with anauction-format listing, and allow a buyer to purchase goods or services,which are also being offered for sale via an auction, for a fixed-pricethat is typically higher than the starting price of the auction.

Store applications 266 allow sellers to group their listings within a“virtual” store, which may be branded and otherwise personalized by andfor the sellers. Such a virtual store may also offer promotions,incentives and features that are specific and personalized to a relevantseller.

Reputation applications 268 allow parties that transact utilizing thenetwork-based trading platform 230 to establish, build and maintainreputations, which may be made available and published to potentialtrading partners. Consider that where, for example, the network-basedtrading platform 230 supports person-to-person trading, users may haveno history or other reference information whereby the trustworthinessand credibility of potential trading partners may be assessed. Thereputation applications 268 allow a user, for example through feedbackprovided by other transaction partners, to establish a reputation withinthe network-based trading platform 230 over time. Other potentialtrading partners may then reference such a reputation for the purposesof assessing credibility and trustworthiness.

Personalization applications 270 allow users of the network-basedtrading platform 230 to personalize various aspects of theirinteractions with the network-based trading platform 230. For example auser may, utilizing an appropriate personalization application 270,create a personalized reference page at which information regardingtransactions to which the user is (or has been) a party may be viewed.Further, a personalization application 270 may enable a user topersonalize listings and other aspects of their interactions with thenetwork-based trading platform 230 and other parties.

In one embodiment, the network-based trading platform 230 may support anumber of marketplaces that are customized, for example, for specificgeographic regions. A version of the network-based trading platform 230may be customized for the United Kingdom, whereas another version of thenetwork-based trading platform 230 may be customized for the UnitedStates. Each of these versions may operate as an independentmarketplace, or may be customized (or internationalized) presentationsof a common underlying marketplace. The latter version may characterizea user's access to the network-based trading platform 230 as originatingfrom a particular country by identifying the country specificpresentation that is selected by the user.

Navigation of the network-based trading platform 230 may be facilitatedby one or more navigation applications 274. For example, a searchapplication allows a user to execute key word searches of listingspublished via the network-based trading platform 230. A browseapplication allows users to browse various category, catalogue, orinventory data structures according to which the listings may beclassified within the network-based trading platform 230.

In order to make listings, available via the network-based tradingplatform 230, as visually informing and attractive as possible, themarketplace applications 250 may include one or more imagingapplications 276 utilizing which users may upload images for inclusionwithin listings. An imaging application 276 also operates to incorporateimages within viewed listings. The imaging applications 276 may alsosupport one or more promotional features, such as image galleries thatare presented to potential buyers. For example, sellers may pay anadditional fee to have an image included within a gallery of images forpromoted items.

Listing creation applications 278 allow sellers to conveniently authorlistings pertaining to goods or services that they wish to transact viathe network-based trading platform 230, and listing managementapplications 280 allow sellers to manage such listings. Specifically,where a particular seller has authored and/or published a large numberof listings, the management of such listings may present a challenge.The listing management applications 280 provide a number of features(e.g., auto-relisting, inventory level monitors, etc.) to assist theseller in managing such listings. One or more post-listing managementapplications 282 also assist sellers with a number of activities thattypically occur post-listing. For example, upon completion of an auctionfacilitated by one or more auction applications 262, a buyer may wish toleave feedback regarding a particular seller. To this end, apost-listing management application 282 may provide an interface to oneor more reputation applications 268, so as to allow the buyer toconveniently to provide feedback regarding a seller to the reputationapplications 268. Feeback may take the form of a review that isregistered as a positive comment, a neutral comment or a negativecomment. Further, points may be associated with each form of comment(e.g., +1 point for each positive comment, 0 for each neutral comment,and −1 for each negative comment) and summed to generate a rating forthe seller.

Dispute resolution applications 284 provide mechanisms whereby disputesarising between transacting parties may be resolved. For example, thedispute resolution applications 284 may provide guided procedureswhereby the parties are guided through a number of steps in an attemptto settle a dispute. In the event that the dispute cannot be settled viathe guided procedures, the dispute may be escalated to a third partymediator or arbitrator.

Messaging applications 288 are responsible for the generation anddelivery of messages to users of the network-based trading platform 230,such messages for example advising users regarding the status oflistings at the network-based trading platform 230 (e.g., providing“outbid” notices to bidders during an auction process or to providepromotional and merchandising information to users).

Merchandising applications 290 support various merchandising functionsthat are made available to sellers to enable sellers to increase salesvia the network-based trading platform 230. The merchandisingapplications 290 also operate the various merchandising features thatmay be invoked by sellers, and may monitor and track the success ofmerchandising strategies employed by sellers.

The network-based trading platform 230 itself, or one or more partiesthat transact via the network-based trading platform 230, may operateloyalty programs that are supported by one or more loyalty/promotionsapplications 292. For example, a buyer may earn loyalty or promotionspoints for each transaction established and/or concluded with aparticular seller, and be offered a reward for which accumulated loyaltypoints can be redeemed.

Marketplace Data Structures

FIG. 11 is a high-level entity-relationship diagram, illustratingvarious tables 263 that may be maintained within the databases 256, andthat are utilized by and support the marketplace applications 250 andpayment applications 252. While the exemplary embodiment of the presentinvention is described as being at least partially implemented utilizinga relational database, other embodiments may utilize other databasearchitectures (e.g., an object-oriented database schema).

A user table 275 contains a record for each registered user of thenetwork-based trading platform 230, and may include identifier, addressand financial instrument information pertaining to each such registereduser. A user may operate as a seller, a buyer, or both, within thenetwork-based trading platform 230. In one exemplary embodiment of thepresent invention, a buyer may be a user that has accumulated value(e.g., commercial or proprietary currency), and is then able to exchangethe accumulated value for items that are offered for sale by thenetwork-based trading platform 230.

The tables 263 also include an items or listings table 291 in which aremaintained item records for goods and services that are available to be,or have been, transacted via the network-based trading platform 230.Each item record within the items table 291 may furthermore be linked toone or more user records within the user table 275, so as to associate aseller and one or more actual or potential buyers with each item record.

A transaction table 277 contains a record for each transaction (e.g., apurchase transaction) pertaining to items for which records exist withinthe items table 291.

An order table 279 is populated with order records, each order recordbeing associated with an order. Each order, in turn, may be with respectto one or more transactions for which records exist within thetransactions table 277.

Bid records within a bids table 281 each relate to a bid received at thenetwork-based trading platform 230 in connection with an auction-formatlisting supported by an auction application 262. A feedback table 283 isutilized by one or more reputation applications 268, in one exemplaryembodiment, to construct and maintain reputation information concerningusers. A history table 285 maintains a history of transactions to whicha user has been a party. One or more attributes tables including an itemattributes table 287 that records attribute information pertaining toitems for which records exist within the items table 291 and a userattributes table 289 that records attribute information pertaining tousers for which records exist within the user table 275.

FIG. 17 shows a diagrammatic representation of machine in the exemplaryform of a computer system 300 within which a set of instructions, forcausing the machine to perform any one or more of the methodologiesdiscussed herein, may be executed. In alternative embodiments, themachine operates as a standalone device or may be connected (e.g.,networked) to other machines. In a networked deployment, the machine mayoperate in the capacity of a server or a client machine in server-clientnetwork environment, or as a peer machine in a peer-to-peer (ordistributed) network environment. The machine may be a server computer,a client computer, a personal computer (PC), a tablet PC, a set-top box(STB), a Personal Digital Assistant (PDA), a cellular telephone, a webappliance, a network router, switch or bridge, or any machine capable ofexecuting a set of instructions (sequential or otherwise) that specifyactions to be taken by that machine. Further, while only a singlemachine is illustrated, the term “machine” shall also be taken toinclude any collection of machines that individually or jointly executea set (or multiple sets) of instructions to perform any one or more ofthe methodologies discussed herein.

The exemplary computer system 300 includes a processor 302 (e.g., acentral processing unit (CPU) a graphics processing unit (GPU) or both),a main memory 304 and a static memory 306, which communicate with eachother via a bus 308. The computer system 300 may further include a videodisplay unit 310 (e.g., a liquid crystal display (LCD) or a cathode raytube (CRT)). The computer system 300 also includes an alphanumeric inputdevice 312 (e.g., a keyboard), a cursor control device 314 (e.g., amouse), a disk drive unit 316, a signal generation device 318 (e.g., aspeaker) and a network interface device 320.

The disk drive unit 316 includes a machine-readable medium 322 on whichis stored one or more sets of instructions (e.g., software 324)embodying any one or more of the methodologies or functions describedherein. The software 324 may also reside, completely or at leastpartially, within the main memory 304 and/or within the processor 302during execution thereof by the computer system 300, the main memory 304and the processor 302 also constituting machine-readable media.

The software 324 may further be transmitted or received over a network326 via the network interface device 320.

While the machine-readable medium 322 is shown in an exemplaryembodiment to be a single medium, the term “machine-readable medium”should be taken to include a single medium or multiple media (e.g., acentralized or distributed database, and/or associated caches andservers) that store the one or more sets of instructions. The term“machine-readable medium” shall also be taken to include any medium thatis capable of storing, encoding or carrying a set of instructions forexecution by the machine and that cause the machine to perform any oneor more of the methodologies of the present invention. The term“machine-readable medium” shall accordingly be taken to include, but notbe limited to, solid-state memories, optical media and magnetic media.Nevertheless, the term “machine-readable medium” shall not be taken toinclude “carrier wave signals.”

Thus, a method and system to locate a storage device have beendescribed. Although the present invention has been described withreference to specific exemplary embodiments, it will be evident thatvarious modifications and changes may be made to these embodimentswithout departing from the broader spirit and scope of the invention.Accordingly, the specification and drawings are to be regarded in anillustrative rather than a restrictive sense.

1. A method to locate at least two storage devices from among aplurality of storage devices, the method including: receiving a requestfor a data item, the request including a data identifier for the dataitem; generating a start number and a step number based on the dataidentifier; locating a first storage device utilizing the start number;if the first storage device is available, reading the data item from thefirst storage device; if the first storage device is unavailable,utilizing the step number and the start number to compute a backupnumber that is utilized to locate a second storage device; and if thesecond storage device is available, reading the data item from thesecond storage device.
 2. The method of claim 1, further including, ifthe second storage device is not available, locating storage devices bycomputing a plurality of backup numbers, each backup number computedutilizing the step number and a previously computed backup number. 3.The method of claim 2, further including recomputing the backup numberbased on a modulo of the plurality of storage devices.
 4. The method ofclaim 3, wherein the start number and the plurality backup numbers areutilized to locate an available storage device from among the pluralityof storage devices in a sequence that is determined by the dataidentifier, the sequence to identify each storage device in theplurality of storage devices without repetition.
 5. The method of claim1, wherein the data identifier is a data number.
 6. The method of claim1, wherein the generating of the start number and the step number isbased on a double hash algorithm.
 7. A method to locate at least twostorage devices to store a data item, the method including: receiving arequest to store the data item, the request including a data identifierfor the data item; generating a start number and a step number based onthe data identifier; locating a first storage device, of a first set ofstorage devices, based on the start number; if the first storage deviceis available, storing the data item on the first storage device; andutilizing the step number and the start number to compute a backupnumber that is utilized to locate a second storage device, of a secondset of storage devices, to store a backup copy of the data item, thebackup number being computed to evenly distribute respective backupcopies of the data item over the second set of storage devices.
 8. Themethod of claim 7, wherein the first storage device is a memory cache.9. The method of claim 7, wherein the data item is a user profile thatis utilized to personalize a web page.
 10. The method of claim 7,wherein the data identifier is stored in a cookie on a client machine.11. The method of claim 10, wherein the data identifier is a datanumber.
 12. The method of claim 7, further including computing the dataidentifier to generate the start number and the step number to evenlydistribute the respective backup copies of the data item over the secondset of storage devices.
 13. A system to locate at least two storagedevices from among a plurality of storage devices, the system including:a receiving module to receive a request for a data item, the requestincluding a data identifier for the data item; and a processing moduleto generate a start number and a step number based on the dataidentifier, the processing module to locate a first storage deviceutilizing the start number, if the first storage device is available,the processing module to read the data item from the first storagedevice, if the first storage device is unavailable, the processingmodule utilizing the step number and the start number to compute abackup number that is utilized to locate a second storage device, if thesecond storage device is available, the processing module to read thedata item from the second storage device.
 14. The system of claim 13,wherein if the second storage device is unavailable, the processingmodule is to compute a plurality of backup numbers to further locatestorage devices, the processing module further utilizing the step numberand the previously computed backup number to compute the next backupnumber.
 15. The system of claim 14, wherein the processing module is torecompute the backup number based on a modulo of the plurality ofstorage devices.
 16. The system of claim 15, wherein the processingmodule is to utilize the start number and the plurality of backupnumbers to locate an available storage device from among the pluralityof storage devices in a sequence that is determined by the dataidentifier, the sequence to identify each storage device in theplurality of storage devices without repetition.
 17. The system of claim13, wherein the data identifier is a data number.
 18. The system ofclaim 13, wherein the processing module is to generate the start numberand the step number based on a double hash algorithm.
 19. A system tolocate at least two storage devices to store a data item, the systemincluding: a receiving module to receive a request to store the dataitem, the request including a data identifier for the data item; and aprocessing module to generate a start number and a step number based onthe data identifier, the processing module to locate a first storagedevice, of a first set of storage devices, based on the start number, ifthe first storage device is available, the processing module to storethe data item on the first storage device, the processing module toutilize the step number and the start number to compute a backup numberthat is utilized to locate a second storage device, of a second set ofstorage devices, to store a backup copy of the data item, the backupnumber selected to evenly distribute respective backup copies of thedata item over the second set of storage devices.
 20. The system ofclaim 19, wherein the first storage device is a memory cache.
 21. Thesystem of claim 19, wherein the data item is a user profile that isutilized to personalize a web page.
 22. The system of claim 19, whereinthe data identifier is stored in a cookie on a client machine.
 23. Thesystem of claim 19, wherein the data identifier is a data number. 24.The system of claim 19, wherein the processing module is to compute thedata identifier to generate the start number and the step number toevenly distribute respective backup copies of the data item over thesecond set of storage devices.
 25. A machine readable medium storing aset of instructions that, when executed by a machine, cause the machineto: receive a request for a data item, the request including a dataidentifier for the data item; generate a start number and a step numberbased on the data identifier; locate a first storage device utilizingthe start number; if the first storage device is available, read thedata item from the first storage device; if the first storage device isunavailable, utilizing the step number and the start number to compute abackup number that is utilized to locate a second storage device; and ifthe second storage device is available, read the data item from thesecond storage device.
 26. A machine readable medium storing a set ofinstructions that, when executed by a machine, cause the machine to:receive a request to store a data item, the request including a dataidentifier for the data item; generate a start number and a step numberbased on the data identifier; locate a first storage device, of a firstset of storage devices, utilizing the start number; if the first storagedevice is available, store the data item on the first storage device;and utilizing the step number and the start number to compute a backupnumber that is utilized to locate a second storage device, of a secondset of storage devices, to store a backup copy of the data item, thebackup number selected to evenly distribute respective backup copies ofthe data item over the second set of storage devices.
 27. A system tolocate at least two storage devices from a plurality of storage devicesto read a data item, the system including: a first means for receiving arequest for the data item, the request including a data identifier forthe data item; and a second means for generating a start number and astep number based on the data identifier; for locating a first storagedevice based on the start number, if the first storage device isavailable; for reading the data item from the first storage device, ifthe first storage device is not available; for adding the step number tothe start number to compute a backup number that is utilized to locate asecond storage device, if the second storage device is available; andfor reading the data item from the second storage device.